Add missing test coverage for ByteBuffer.allocateDirect().

This method was called as a side-effect of some other tests, but
never as part of a dedicated / comprehensive test. For example,
the case of a negative argument was not covered by tests. This CL
adds such coverage.

This CL also tweaks a couple of assertContentEquals() helper
methods to be static.

Test: testAllocateDirect() passes.

Bug: 35910877

Change-Id: I6807383d0c47d3b8e6d3c8b15fc3ba0432a92b3d